1. Field of the Invention
The present invention relates to a monopole antenna and electronic device adding a coupling element to have enough bandwidth and smaller size.
2. Description of the Prior Art
Due to the prosperous development of wireless communications in recent years, more and more information is transmitted through wireless networks and thus demand for wireless communications increases. Moreover, advances in laptop and pad computer technology also increase requirements for compact products, including reduced size antennas.
In general, a consumer communications device utilizes a dipole antenna or a monopole antenna to perform wireless signal transmission and reception. The dipole antenna is composed of two bent metal lines with a half wavelength of a radiating frequency, but the size is too large for some portable devices and its differential feed-in results in unstable antenna performance. A monopole antenna is derived from the dipole antenna. The monopole antenna has only one metal line as a radiator with the other metal line replaced by a large ground. The large ground forms a mirror effect, so the monopole antenna has an antenna pattern similar to that of the dipole antenna. In such a situation, the monopole antenna has a size smaller than the dipole antenna.
Please refer to FIG. 1, which is a schematic diagram of a traditional monopole antenna 10. The monopole antenna 10 is composed of a radiating element 102 made of a metal line vertically formed on a grounding element 100, and a radio-frequency signal is fed-in to the monopole antenna 10 via a feed-in element 104. The monopole antenna 10 is made by cutting the radiating element 102 to a length equal to a quarter wavelength of the radiating frequency. Due to the simple physical characteristics of the monopole antenna, it is easy to design and has a low manufacturing cost. Hence, the monopole antenna is widely used for the electronic products with wireless communications functionality.
However, the traditional monopole antenna lacks design flexibility because there is only one radiating band centered on the radiating frequency. The traditional monopole antenna requires a size (length) equal to a quarter wavelength of the radiating frequency and decreasing the size within a limited antenna space is difficult. Therefore, finding solutions to the above problem have become a goal of the wireless communications industry.